This question is about general tips, not just openings
Is there any tips for this variant KOTH, which requires the player to bring the king to the center before the opponent, or by checkmating the opponent?
E.g. do not bring your king out too early and how to counter it if the opponent plays it
General tips for KOTH variant.
Take firm control of the center squares in the opening.
Do it by attacking the squares, but avoid placing pawns there.
Keep the center squares free of your pawns, so your king gets there later.
For example, play c2-c4 and thereby control the d5-square.
Play g3, Bg2, and control e4 and d5.
If you notice an early king march from your opponent, then go for an all out attack on that king and thereby win some material.
Do not go for an early king march yourself, castle early, then play standard chess.
Take control of the file that the opponent's king must pass to get to the center. For example, if opponent castles kingside, take control of the e-file with all heavy pieces. If possible, f-file and e-file control is strong.
Gradually increase your control in the center, then create a path for your king to enter, e.g. g1-f2-e3 and prepare your pieces so they can sac themselves to free the e4 or d4 squares.
Make sure you have ways to drive your opponent's king back. Break any path they try to create.
Keep most pieces on the board, go for a long middlegame.